The present invention relates generally to methods and systems for inspecting defects on wafers, masks, and reticles. More particularly, the present invention relates to optical inspection systems and techniques.
In a conventional optical inspection system, defects are detected by subtracting a reference image from a test image to produce a difference image. The test image is an optical image of an area on the photomask. The reference image may be an optical image of a similar area on an identical die or on the same die or rendered from a design database. The grayscale residues, i.e., portions of the difference image having a value other than zero, may represent defects in the inspected sample.
Conventional optical metrology techniques use intensity based or scattering based systems. With intensity based systems it is difficult to detect defects sitting in dark structures. Examples of dark structures include the trenches of high aspect ratio devices. Because the background is darker, slight perturbations may not easily be discriminated against the background. Furthermore, dark structures provide relatively low signal levels.
For example, and as illustrated in FIG. 1, the intensity 102 of the reflected signal from a contact hole 104 having an aspect ratio of 3:1 is very low in the vicinity of the hole 104. The normalized intensity 106 for the surface areas 108 outside the contact hole 104 is shown in the illustration to be considerably larger. This plot illustrates the reflected intensity profile under normal coherent illumination. Thus, defects residing on the trench floor would be difficult to discriminate against such a low intensity (dark) background as illustrated by intensity level 102. Moreover, other background noise, such as from misalignment between images for example, may mask slight differences in intensity attributable to the defect.
Laser scattering metrology techniques also rely on the interaction between the illumination and the defect structure, and therefore experience similar difficulties in identifying defects in high aspect ratio structures, i.e., high aspect ratio inspection (“HARI”). Device miniaturization trends are expected to exacerbate this problem. As smaller geometries are used, the resulting smaller structures with smaller HARI will be difficult to detect with current tools. Scanning electron microscopy (SEM) techniques are capable of inspecting HARI defects but are unsuitable for inspection inline. SEM techniques are slow, and require that wafers must be taken off line for the inspection.
As described in United States Nonprovisional application Ser. No. 10/672,298, entitled “METHOD AND APPARATUS USING INTERFEROMETRIC METROLOGY FOR HIGH ASPECT RATIO INSPECTION” naming Hwang et al. as inventors, submitted by at least one of the same inventors of the present invention, phase based techniques and inspection systems are capable of identifying subtle defects, i.e., defects which have very little intensity. But phase measurement is not suitable for detecting all defects. Conventional intensity based inspection systems, for example, are more efficient in identifying many defects such as, large defects or other defects producing a large intensity signal. Phase based inspection techniques are especially suitable for small, subtle defects but aren't ideal in the presence of a signal having a strong intensity. Phase measurements in such an application will create an abundance of problems. A large defect may yield only a small phase defect signal. Phase repeats every cycle (i.e., every angular change of 2 pi radians), thus an optical path difference exceeding one cycle may show up as only a small phase difference. Moreover, in identifying such defects, intensity based systems have lower throughputs and are more sensitive to pattern noise.
Accordingly, what is further needed is an inspection system capable of providing conventional intensity based (i.e., brightfield) inspection for defects generating sufficient signal intensities and further capable of providing inspection of defects which generate low intensity signals.